driver core: Add iommu_group tracking to struct device

IOMMU groups allow IOMMU drivers to represent DMA visibility
and isolation of devices. Multiple devices may be grouped
together for the purposes of DMA. Placing a pointer on
struct device enable easy access for things like streaming
DMA programming and drivers like VFIO.

Signed-off-by: Alex Williamson <[e-mail]>
Acked-by: Greg Kroah-Hartman <[e-mail]>
Acked-by: Benjamin Herrenschmidt <[e-mail]>
Signed-off-by: Joerg Roedel <[e-mail]>

include/linux/device.h

···
 struct bus_type;
 struct device_node;
 struct iommu_ops;
+struct iommu_group;
 
 struct bus_attribute {
 	struct attribute	attr;
···
 	const struct attribute_group **groups;	/* optional groups */
 
 	void	(*release)(struct device *dev);
+	struct iommu_group	*iommu_group;
 };
 
 /* Get the wakeup routines, which depend on struct device */